The invention relates to the packaging of polycrystalline silicon.
Polycrystalline silicon, referred to hereinafter as polysilicon, serves, inter alia, as a starting material for the production of electronic components and solar cells.
It is obtained by thermal decomposition of a silicon-containing gas or of a silicon-containing gas mixture. This operation is referred to as deposition from the gas phase (CVD, chemical vapor deposition).
On a large scale, this operation is implemented in what are called Siemens reactors. In this case, the polysilicon is obtained in the form of rods. The polysilicon rods are generally comminuted by means of manual processes.
A number of machine processes are known, in which manually precrushed coarse polysilicon chunks are comminuted further using customary crushers. Mechanical crushing processes are described, for example, in U.S. Pat. No. 8,021,483 B2.
U.S. Pat. No. 8,074,905 discloses an apparatus comprising a device for feeding coarse polysilicon chunks into a crusher system, the crusher system and a sorting system for classification of the chunk polysilicon, wherein the crusher system is provided with a controller which allows variable adjustment of at least one crushing parameter in the crusher system and/or at least one sorting parameter in the sorting system.
For applications in the semiconductor industry and solar industry, chunk polysilicon with a minimum level of contamination is desirable. In order to accomplish this, various purification processes are also used.
US 2010/0001106 A1 describes a process for producing high-purity classified chunk polysilicon, in which a polysilicon from the Siemens process is comminuted and classified by means of a device comprising comminution tools and a screening device, and the chunk polysilicon thus obtained is cleaned by means of a cleaning bath, wherein all of the comminution tools and the screening device have a surface which comes into contact with the polysilicon made of a material which contaminates the chunk polysilicon only with those extraneous particles which are subsequently removed selectively by the cleaning bath.
Silicon dust adhering to the chunks is also regarded as contamination, since it reduces the yield in crystal pulling.
US 2010/0052297 A1 discloses a process for producing polycrystalline silicon, comprising crushing of polycrystalline silicon deposited on thin rods in a Siemens reactor into chunks, classifying the chunks into size classes from about 0.5 mm to greater than 45 mm and treating the chunks by means of compressed air or dry ice in order to remove silicon dust from the chunks, with no wet chemical purification.
However, the polycrystalline silicon has to be packaged after the comminution steps and any cleaning or dedusting performed before being transported to the customer.
Accordingly, it should be ensured that the packaging is effected with a minimum level of contamination.
Typically, chunk polysilicon for the electronics industry is packaged in 5 kg bags with a weight tolerance of +/−max. 50 g. For the solar industry, chunk polysilicon in bags with a weight of 10 kg and a weight tolerance of +/−max. 100 g is customary.
Tubular bag machines suitable in principle for packaging of chunk silicon are commercially available. A corresponding packaging machine is described, for example, in DE 36 40 520 A1.
Chunk polysilicon is, however, a sharp-edged, non-free-flowing material having a weight of the individual silicon chunks of up to 2500 g. Therefore, in the course of packaging, it should be ensured that the material does not penetrate the customary plastic bags in the course of filling, or in the worst case even completely destroys them.
In order to prevent this, the commercial packaging machines have to be modified in a suitable manner for the purpose of packaging polysilicon.
U.S. Pat. No. 7,013,620 B2 discloses an apparatus for inexpensive, fully automatic transportation, weighing, portioning, filling and packaging of a high-purity chunk polysilicon, comprising a conveyor channel for the chunk polysilicon, a weighing device for the chunk polysilicon, connected to a hopper, deflecting plates made from silicon, a filling device which forms a plastic bag from a highly pure plastic film, comprising a deionizer which prevents static charging and hence particle contamination of the plastic film, a welding device for the plastic bag filled with chunk polysilicon, a flowbox which is fitted above the conveyor channel, weighing device, filling device and welding device and which prevents contamination of the chunk polysilicon with particles, a conveyor belt with a magnetically inductive detector for the welded plastic bag filled with chunk polysilicon, wherein all components which come into contact with the chunk polysilicon are sheathed with silicon or clad with a highly wear-resistant plastic.
DE 103 46 881 A1 discloses a system for filling and sealing open plastic sacks, equipped with a filling machine comprising a rotor which can be driven so as to rotate about a vertical axis and is equipped with a plurality of filling devices on which the plastic sacks to be filled can be hung, and in which the filling devices are assigned welding units for production of the closure seams after the removal of the filled plastic sacks from the filling devices, and the system is also equipped with a linear discharge belt to transport the filled plastic sacks away from the filling machine, wherein the rotor of the filling machine can be driven at constant speed and is equipped with closure seam weld units assigned to the filling stubs, and the individual welding devices on the rotor of the filling machine are also assigned pivotable sack support devices which accept the plastic sacks to be removed from the filling devices immediately after the production of the closure seams by the welding devices and pass them onto a discharge belt which can be driven at the peripheral speed of the rotor and is arranged so as to be stationary and tangential thereto.
It has been found that, in the case of such apparatuses, jamming of the silicon chunks in the filling device often occurs. This is disadvantageous since it results in increased shutdown times for the machine.
Puncturing of the plastic bag also occurs, which likewise leads to a shutdown of the plant and to contamination of the silicon.
It has also been found that, during the packaging of chunks of a particular size class, for example chunks of 20 to 60 mm, unwanted smaller silicon particles or chunks also arise. The proportion of such unwanted particles for such chunk sizes is 17,000-23,000 ppmw.
Hereinafter, all chunks or particles of silicon having such a size that they can be removed by a mesh screen having 8 mm×8 mm square meshes are to be referred to as fines. Fines are undesirable to the customer, since they adversely affect the customer's operations. If the fines are removed by the customer, for example by screening, this means an increased level of cost and inconvenience.
As well as the automatic packaging of polycrystalline silicon, such as that according to U.S. Pat. No. 7,013,620 B2, manual packaging of the polycrystalline silicon in plastic bags is also an option. Manual packaging can distinctly reduce the fines fraction, for the abovementioned 20-60 mm chunk size from 17,000 ppmw down to 1400 ppmw.
However, manual packaging means a high level of complexity and increased personnel costs. Therefore, manual packaging is not an option for economic reasons. In addition, it would be desirable to reduce the fines fraction even further than is achievable by manual packaging.
It was therefore an object of the invention to automatically package polycrystalline silicon and to reduce the fines fraction which arises to an extremely low level. It was also an object of the invention to provide an apparatus suitable for this purpose.